1. Field of the Invention
The invention is related to the field of communications and, in particular, to test systems and test methods for determining the cause of an extended off-hook condition on a subscriber line.
2. Statement of the Problem
In telephony, a subscriber line (also referred to as the local loop) is the physical link that connects a demarcation point of a customer premises to the edge of the carrier network. One end of the subscriber line connects to one or more telephones at the customer premises, and the other end of the subscriber line connects to a switch housed in a central office (CO) or in a remote unit controlled by the main switch in a central office. Traditionally, the subscriber line is provisioned as a single twisted pair in support of voice communications.
The switch in the central office is able to detect when one or more of the phones at the customer premises goes “off hook”, presumably to place a call. To perform the detection functionality, the switch applies a voltage over the subscriber line through a battery voltage that is traditionally −48 volts DC. When all of the telephones that are connected to the subscriber line are “on-hook” (i.e., not being used), then there is an open circuit at the customer premises end of the subscriber line that the switch is able to detect. When one of the telephones is taken “off-hook” (i.e., the phone has been activated for use), the telephone in use completes or closes a circuit over the subscriber line by connecting one of the twisted pair wires to the other through a resistance of the internal circuitry of the telephone. In telephony, the term “off-hook” traditionally refers to the condition that exists when a telephone or other customer premises equipment is in use (i.e., during dialing or communicating). The switch is able to detect when the subscriber line changes from an open circuit to a completed circuit, and provides a dial tone over the subscriber line.
There may be instances where an off-hook condition is sensed by the switch for an extended period of time. The extended off-hook condition may be the result of a handset accidently being knocked off of a telephone. In such a case, the switch would provide re-order tones over the subscriber line. The extended off-hook condition may alternatively be the result of a fault on the subscriber line, such as a short circuit somewhere along the length of the subscriber line. A short circuit refers to the condition that exists when the twisted pair wires of the subscriber line are electrically shorted across one another. When a short circuit occurs, the switch will detect a change in resistance on the subscriber line from an open circuit to a much lower resistance. In the case of a short circuit, a technician would need to be contacted to locate and remove the short condition.
To test for these extended off-hook conditions, the central office also includes a test system that connects to the subscriber lines. If an extended off-hook condition is detected on a subscriber line, then the test system performs a test method to determine whether the extended off-hook condition is the result of an off-hook telephone or a short circuit on the subscriber line. In one exemplary test method presently used, the test system measures two resistances over the subscriber line at two different currents, and then divides the lower resistance measurement by the higher resistance measurement to determine whether the resulting percentage is less than or greater than a threshold percentage. Some test methods use 15% as the threshold, and others use 50% as the threshold.
For an off-hook telephone, the circuit of the subscriber line comprises the resistances of the twisted pair wires and the resistance of the telephone that is off hook. Most telephones have a non-linear resistance, meaning that the resistance of the telephone decreases as the current (or voltage) increases. In a short circuit, the circuit of the subscriber line comprises the resistances of the twist pair wires up to the location of the short. The resistances of the twisted pair wires are substantially linear. For a linear resistance, an incremental increase in voltage (or current) results in an equivalent incremental increase in current (or voltage).
According to the presently used test methods, the test system determines whether the measured resistances indicate a linear resistance or a non-linear resistance by calculating the percentage of the lower resistance measurement and the higher resistance measurement. A linear resistance on the subscriber line will result in a percentage near 0%, which indicates that the extended off-hook condition is the result of a short circuit on the subscriber line. A non-linear resistance on the subscriber line will result in a much higher percentage (e.g., 15% to 50%), which indicates that the extended off-hook condition is the result of an off-hook telephone.
One problem for the present test systems and methods is that that the amount of non-linearity of telephones is decreasing. Older rotary telephones have a non-linearity of 500 Ohms or more, while newer telephones have a non-linearity of around 100 Ohms. The present test methods using the percentage calculation may not be able to distinguish between an off-hook telephone and a short circuit when newer telephones are used. Also, the test systems are typically fabricated using lower-cost components that have higher manufacturing tolerances (e.g., 10% or more). Due to the manufacturing tolerances, the traditional test systems and methods may not be able to effectively distinguish between an off-hook telephone and a short circuit on a subscriber line.